Method of making porous zinc structures



April 30, 1968 Original Filed March 30, 196E POROS/TY 0F .Sl/VTERED Z/NCSTRUCTURES {96 F. J. KELLY ET AL 3,380,822

METHQD OF MAKING POROUS ZINC STRUCTURES 3 Sheets-Sheet 1 -3 OPT/MALCOMPACT/N6 PRESSURE X /0 (psi) /0 l2 l4 l6 l8 lllll NH Cl FILL/N6 AGENTCONTENT m1 INVENTORS.

FRANCIS JOHN KELLY FRANC/SZEK PRZYBYLA April 30, 1968 J KELLY ET ALMETHOD OF MAKING POROUS zmc STRUCTURES I5 Sheets-Sheet 2 Original FiledMarch 30, 1965 CELL EXPERIMENTAL PLANT CELL (cue A/VODE/ TIME {HOURS ..3COMPACT/N6 PRESSURE X 10 (PSI) wLdw w=ww=wa 988.676 5 at Wtbkbbmtm Q\ \Nb6 xkadmsl INVENTORS:

FRANCIS JOHN KELLY FRANC/SZEK PRZYBYLA BY: 6auanagA & Worman A ril 30,1968 Original Filed March 30, 1965 CELL TERMINAL VOL TAGE CELL TERMINALVOLTAGE F. J. KELLY ET AL METHOD OF MAKING POROUS ZINC STRUCTURES 3Sheets-Sheet 3 EXPERIMENTAL CELL PLANT CELL EXPERIMENTAL CELL PLAN TCELL IN VENTORS.

FRANCIS JOHN KELLY FRANC/SZEK PRZYBYLA 8y: Cauanag 8 770F023"- UnitedStates Patent 3,380,822 METHOD OF MAKING POROUS ZINC STRUCTURES FrancisJohn Kelly and Franciszek Przybyla, Toronto,

Ontario, Canada, assignors to Mallory Battery Company of Canada Limited,Clarkson, Untario, Canada Original application Mar. 30, 1965, Ser. No.443,986. Divided and this application Feb. 14, 1967, Ser. No. 616,107

4 Claims. (Cl. 75-214) ABSTRACT OF THE DISCLOSURE A method of sinteringparticles of zinc having an oxide coating thereon, into a porousstructural body, comprising the steps of admixing such particles of zincwith an amount of sublimable ammonium halide filler particles in vastexcess of that required to react with the oxide coating, compacting theadmixture at an elevated pressure, heating the compacted body to asintering temperature less than the melting temperature of zinc, wherebya portion of the filler reacts with the oxide coating forming asublimable reaction product, and the unreacted portion of the fillersublimes, controlling the pressure on the body during the heating, to apredetermined pressure less than atmospheric pressure to accelerate andeffect substantially complete sublimation of the filler and thesublimable reaction product.

This application is a division of application Serial No. 443,986, filedMarch 30, 1965, now Patent No. 3,348,976.

This invention relates to a method of making a sintered porous zincstructure.

There is a need for metal structures of high porosity i.e. high surfacearea volume ratio and of sound structural integrity for application inelectrode structures for electrical batteries and the like. Usefulelectrode materials such as zinc and cadmium cannot be formed intoporous bodies by sintering. It is known that an oxide film on copper oron silver can be removed by hydrogen reduction at less than the meltingpoint of copper or silver. The sintering of copper or silver powders toform relatively dense porous structures is not difficult. However oxidefilms on zinc and cadmium are not removed by hydrogen reduction at atemperature below the melting point of such metals. It is for thisreason that to date there are no commercially known techniques for theproduction sintering of zinc and cadmium metal powders.

The formation of sintered porous metal structures by the utilization ofcarbon or hydrocarbon fillers adapted to determine the porosity of thestructure and to be burnt out during sintering is known. However theemployment of hydrocarbon filler materials for such purpose will tend tocontaminate the resulting structure with residue. Not all of theproducts of combustion can be removed in this manner. Furthermore thecombustion disrupts the structure during sintering or prior to sinteringas the temperature is raised. Thus for example it is known that amixture of carbon particles and iron oxide particles compacted andsubjected to heat may swell and burst in the formation of sponge iron.Filler material combustion is an old technique and is subject to manylimitations and in particular limitations as to the degree of porositywhich may be achieved. Porosity in such prior structures is limitedgenerally to substantially less than fifty percent and is controlledmainly by compacting pressures.

It is a main object of this invention to provide a method of makingporous metal structures and method of making same in which the porosityis substantially independent Patented Apr. 30, 1968 ice of compactingpressure relative to the structural integrity of the structure itself.

It is a further object of the invention to provide a method of making aporous metal structure of full structural integrity by vacuumsublimation of a sublimable filler mixed with a powdered metal todetermine the porosity thereof and adapted to be removed completely bysublimation at a temperature near to and less than sinteringtemperature.

Other objects of the invention will be apparent from the followingspecification taken in conjunction with the accompanying drawings.

In the drawings:

FIGURE 1 is a curve disclosing porosity as a function of filling agentcontent according to the invention.

FIGURE 2 is a set of curves disclosing the porosity of a porousstructure herein as a function of compacting pressure.

FIGURE 3 records the life by a voltage time curve of a zinc structureherein used as a battery anode compared wi.h a conventional batteryanode at low temperature.

FIGURE 4 is a similar voltage time curve to that of FIGURE 3 for adifferent style of battery structure but again indicating thesuperiority of a cell employing the anode of the invention.

FIGURE 5 compares the cyclic discharge life of two battery cells one ofwhich is conventional and the other of which embodies an anode formed ofa zinc structure made according to the invention.

Zinc and cadmium particles once exposed to air are coated with a thinsuperficial layer of oxide. In order to bond the particles attemperatures below the melting point by sintering it is mandatory thatthe oxide layer be removed. This layer however cannot be reduced byhydrogen at temperatures below the melting point of the metal.

It is therefore necessary to remove the oxide by a reaction other thanreduction. According to the invention and in the case of zinc successfulchemical reactions appear to be of the form:

in which X represents a chloride, bromide or iodide ion. This reactionproceeds at a temperature considerably lower than the melting point ofzinc. According to the invention the reaction product as well as anyunchanged ammonium halide can be removed by sublimation under reducedpressure at such reaction temperature. At this and slightly highertemperatures the cleansed metal particle surfaces adhere in apreliminary sintering thus maintaining the particle structureestablished during the compacting process. Finally still under vacuumthe zinc particle bonding is reinforced at the sintering temperature.

An essential feature of this method is that a preliminary bonding mustbe made between adjoining zinc particles concurrently with removal ofthe filling agent. This requires a sublimation temperature herein nearthe temperature at which sintering commences. This has been establishedas approximately C. below sintering temperature. At this temperature theammonium halides sublime slowly enough for a mixed pellet to maintainits integrity.

Deformation of individual zinc or cadmium particles begins to occurbeyond 320 C. the maximum recommended temperature at which sintered zincand cadmium structures with predetermined size and shape can beobtained.

Only a very small quantity of ammonium halide is required to remove azinc or cadmium oxide film. The vast excess used in this process servesmerely as filling agent. Ammonium halides are especially suitable asfilling agents since they can be removed by vacuum sublimation attemperatures at which presinterin'g of zinc and cadmium TABLE 1 Porosityof Filling agent content 1 (Percent w./w.)

sintered zinc pellet, Percent NlI Cl NlIBr NHI 1 Compacting pressureslisted in Table 2.

As shown in Table 1 and FIGURE 1 regulation of the filling agent contentprovides a very sensitive means of producing any desired porosity. As aconsequence of their diifering densities markedly different weightpercentages of the various halides excepting fluorides are used inproducing any one specified porosity.

The influence of compacting pressure on porosity is minor as can be seenfrom the pressure porosity graph of FIGURE 2. A twofold increase incompacting pressure (from 9,000 to 19,000 p.s.i.) reduces the porosityobtained by only 2%.

It is therefore necessary according to this invention to regulate theporosity by filling agent content rather than by compacting pressure. Anoptimal pressure can be chosen to yield pellets of suitable greenstrength. Experimentally determined optimal pressures are listed inTable 2.

TABLE 2 Filling agent content, Compacting pressure percent: (p.s.i.)

EXAMPLE 1 An intimate mixture consisting of 6.00 g. of zinc powder (325mesh, 3%; +325, 7%; +200, 30%; +100, 30%; +60, 30%; +20, 0%) and 2.51 g.of ammonium chloride (-60 mesh, 100%) was compressed at 19,000 p.s.i.into a pellet, diameter 2.55 om., height 0.53 cm. The pellet was placedin an evacuated (2 mm. Hg) glass sublimation apparatus and thetemperature was raised quickly to 220 C. Ammonium chloride was sublimedquantitatively in the temperature range 220-240" C. The temperature wasraised slowly to 280 C. and held at this temperature for approximatelyfifteen minutes. The apparatus still under evacuation was allowed tocool to room temperature and the pellet removed. The pellet resultinghad a porosity of 69.5% (as measured by the volumetric method) anddimensions 2.54 cm. diameter 0.52 cm. height. The loss of zinc duringthe entire operation amounted to 2%.

EXAMPLE 2 The same mixture was compressed identically and the fillingagent sublimed in the same way as in Example 1. The temperature was thenraised quickly from 240 C. to 320 C. and the apparatus immediatelyallowed to cool. The pellet resulting was identical with that obtainedin Example 1.

EXAMPLE 3 An intimate mixture consisting of 6.00 g. zinc powder (325mesh, 3%; +325, 7%; +200, 30%; +100, 30%;

60, 30%; +20, 0%) and 4.1 g. ammonium bromide (60 mesh, was compressedat 14,000 p.s.i. into a pellet diameter 3.00 cm. height 0.38 cm. Thepellet Was placed in an evacuated (2 mm. Hg) sublimation apparatus andthe temperature raised to 220 C. Ammonium bromide was sublimedquantitatively in the ternperature range 220-240" C. The temperature wasraised slowly to 300 C. and held at this temperature for about tenminutes. The apparatus still under evacuation was allowed to cool toroom temperature and the pellet was removed. The resulting pellet had aporosity of 68% and dimensions 2.99 cm. diameter and 0.37 cm. height.The loss of zinc during the entire operation amounted to 2%.

EXAMPLE 4 An intimate mixture of 6.00 g. zinc powder (sieve analysis asin Example 3) and 2.45 g. ammonium iodide (60 mesh, 100%) was compressedat a pressure of 12,000 p.s.i. into a pellet, diameter 3.00 cm. height0.29 cm. The pellet was placed in an evacuated (2 mm. Hg) sublimationapparatus and the temperature raised to 200 C. Ammonium iodide wassublimed quantitatively in the temperature range 220260 C. Thetemperature was raised slowly to 300 C. and held at this temperature forabout ten minutes. The apparatus still under vacuum was allowed to coolto room temperature and the pellet removed. The resulting pellet had aporosity of 58%. The loss of zinc during the entire operation amountedto 2%.

From the foregoing it will be apparent that the sintered porous metalstructure of the invention is of entire structural integrity i.e. eachand every particle is bonded to at least one adjacent particle to definean open three-dimensional network having communicating voids extendingbetween the particles through the structure. In practice while the metalhalide formed on the surface of the metal particles may be evaporatedcompletely i.e. by controlled temperatures over a period of hours it isto be observed that such metal halides are soluble salts entirelycompati ble in small amounts with the chemistry of electricalapplications as anodes of various kinds. The time of sinteringsufiicient to impart a desired degree of structural integrity may leavetrace amounts of soluble salts within the structure. A structure of theinvention may thus be identified if made of substantially uniform sizedirregularly shaped particles by a photographic appearance in section andmay sometimes contain a trace amount of a soluble filler material ormetal salt. Having regard to the unavailability of porous metalstructures of entire structural integrity of zinc heretofore theexistance of a porous structure of such metal containing at least atrace amount of a soluble salt will indicate this invention as itssource.

Thus the invention concerns methods of forming metal structures ofentire structural integrity from particles of zinc which particles arenot ordinarily adapted to be sintered due to the nature of the oxidefilm or coating formed thereon and present in normal atmosphere. A bodymay be formed according to this invention of a predetermined porositydetermined by the volume of filler material employed for compaction withthe metal particles. The filler is a soluble crystalline ammoniumbromide, chloride or iodide. The filler employed must be characterizedby a sublimation temperature not greater than the temperature at whichsintering of the metal particles is to be achieved. The inventionembodies the increasing of the rate of sublimation by controllingpressure to a pressure less than atmospheric. In the vacuum sublimationtechnique or reactive sintering as practised herein the filler materialswill sublime at less than sintering temperature of the metal and at notless than about 180 C. below such sintering temperature. If the fillersublimes at too low a temperature, i.e. at too great a temperature,separation below the melting point of the metal i.e. below sinteringtemperature, the structure will not be suificiently supported duringsintering. Preferably therefore, the filler material should not sublimeof the metal halides formed therefrom vaporize or distill at less thanabout 200 C. below the satisfactory sintering temperature at whichsulficient welding takes place. The sintering temperature may besomewhat less than melting temperature especially where a fresh metalsurface is provided such as by the method and technique of theinvention. This will explain a case where the sublimation temperature ofthe filler may be substantially below the melting temperature and yetdeliver a porous body of high structural integrity according to theinvention.

Porous metal structures of the invention demonstrate a very large activesurface area. In FIGURE 3 an anode was set up in an experimental cellthe anode being formed of zinc particles reactively sintered accordingto the invention with a filler of ammonium chloride. The experimentalcell and the conventional plant type cell with which it Was comparedwere discharged current drain of 5 milliamperes at 1.25 volts at atemperature of 20 C. The conventional plant cell exhausted its practicallife in less than three hours whereas the experimental cell maintainedits practical life for substantially five times this useful life.

In FIGURE 4 a further comparison test with a similar type anode materialto a different type of cell structure was compared with a conventionalplant cell of the same structure at a continuous discharge 5milliamperes at 1.3 volts at a temperature of 20 C. The useful life ofthe plant cell expired in less than 1% hours whereas the experimentalcell having an anode of the invention continued its useful life for morethan 11 hours.

In FIGURE 5 is demonstrated an application of a zinc porous structure toan anode of an experimental cell compared with a conventional plant cellof the same design in all other respects at a temperature of 21 C. and aload of 14.5 ohms. This was an intermittent cyclic discharge of aprogram of 15 seconds on (to 0.8 volts) 45 seconds off. The usefulcyclic discharge life of the anode of the invention again demonstratesits superiority in providing substantially four times the cyclic life ofthe conventional cell.

Vacuum reactive sintering of zinc of this invention involves thesublimation of the filler material at a rate which is accelerated by theuse of vacuum. The fillers of the invention will sublime very slowly atnormal temperature and pressure. A reduction of pressure combined withan increase in temperature greatly accelerates this property of thefiller crystals. While most of the filler is sublimed, that next thesurface of the zinc particles enters into chemical reaction with theoxide filler on the surface of the particles to provide zinc halide.Such metal salts are characterized by a predetermined vapour pressure atthe melting temperature of the salt. The reduction of pressuresaccelerates the vapourization process and renders such mechanismpractical.

While zinc and cadmium bromide, chloride and iodide vaporize at atemperature less than the melting temperature of the corresponding zincmetal and are thus particularly suitable to the invention mechanism, itis to be noted that zinc fluorides formed by oxide reaction with areadily sublimable ammonium fluoride have melting temperatures muchhigher than the melting temperatures of these metals. Accordingly, whileall halides are workable the fluoride metal salt will remain in thestructure as a substitute for oxide film and will be present insubstantial quantity but readily soluble form. In many applications thismay not be a disadvantage since the structure will nevertheless beporous due to sublimation of the major portion of the filler and morethan a trace amount of water soluble metal salt may be dissolved out ofthe structure if desired.

While the invention may find a variety of uses it is evident thatbattery cells having structurally superior anodes may be fabricated toprovide many times the useful battery life of that experienced withbattery cells of the prior art by this invention.

It is intended that the disclosures herein set forth should not beconstrued in any limiting sense other than that indicated by the scopeof the following claims having regard to the art of forming sinteredporous bodies of zinc described herein.

What we claim as our invention is:

1. The method of sintering particles of zinc having an oxide coatingthereon to form a body of entire structural integrity of predeterminedporosity and comprising: admixing such particles of zinc with an amountof sublimable ammonium halide filler particles in vast excess of thatrequired to react with said oxide coating, compacting said admixture atan elevated pressure to provide a compacted body of predetermined shapeand suitable green strength; heating said compacted body to a sinteringtemperature less than the melting temperature of zinc, whereby a portionof said filler reacts with said oxide coating forming a sublimablereaction product, and the unreacted portion of said filler sublimes,and; controlling the pressure on said body during said heating to apredetermined pressure less than atmospheric pressure to accelerate andeffect substantially complete sublimation of said filler and saidsublimable reaction product from said body, said filler therebymaintaining structural support of said particles during heating up tosubstantially the temperature at which surfaces of said particles arejoined by sintering.

2. The method according to claim 1 in which the filler comprisesparticles of Water soluble crystalline ammonium halide of a sizegenerally corresponding to the size of the zinc particles.

3. The method according to claim 1 in which the body is maintained at atemperature and under vacuum for vacuum sublimation at greater thanabout 200 C. below sintering temperature but less than sinteringtemperature and thereafter the temperature thereof raised to sinteringtemperature.

4, The method according to claim 1 in which the body is maintained at atemperature and under vacuum for vacuum sublimation at greater thanabout 200 below sintering temperature but less than sinteringtemperature and thereafter the temperature thereof raised to sinteringtemperature, and the pressure is controlled to a vacuum of between aboutone and about five millimetres of mercury.

References Cited UNITED STATES PATENTS 1,642,348 11/ 1927 Williams 2221,988,861 1/ 1935 Thorausch 75-222 3,050,386 8/1962 Von Dohren 752223,142,892 8/ 1964 Powell 75222 3,251,684 5/ 1966 Spacil 75224 3,287,11211/ 1966 Blaha 75222 FOREIGN PATENTS 1,180,948 11/1964 Germany.1,013,375 12/ 1965 Great Britain.

OTHER REFERENCES Cox, Vacuum sintering, Metal Industry, Sept. 2,

CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

A. I. STEINER, Assistant Examiner.

